The reality of quicksand is very different that in the movies. In particular, the Wikipedia article linked above says:

It is impossible for a human to sink entirely into quicksand, due to the higher density of the fluid. Quicksand has a density of about 2 grams per cubic centimeter, whereas the density of the human body is only about 1 gram per cubic centimeter. …

…

Quicksand may be escaped by slow movement of the legs in order to increase viscosity of the fluid, and rotation of the body so as to float in the supine position (lying horizontally with the face and torso facing up).

Technically, quicksand is a “shear thinning non-Newtonian fluid.”